To construct a Hubble diagram for QSOs, the required observational parameters are the rest-frame flux density at both 2500 Å and 2 keV (see equation REF ) and the redshift.
A detailed spectroscopic analysis at both the UV and X-ray energies is preferred but can be carried out only for a relatively small number of sources. Therefore, the currently published QSO sample still heavily relies on broadband photometry at both UV and X-rays, and so the computation of both \(F_{\rm X}\)  and \(F_{\rm UV}\)  is through their photometric rest-frame SED.
To compile the QSO SEDs, multiwavelength data from radio to UV should be considered, where Galactic reddening must be taken into account in the optical/UV by utilising the selective attenuation of the stellar continuum (e.g., [1]}), along with the relative Galactic extinction (e.g., [2]}) for each object. Most of the relevant broadband information, as well as the spectroscopic redshifts, are compiled in the online QSO catalogues of several surveys such as SDSS [3]}. The reader may refer to [4]}, [5]}, [6]} for a detailed description of how a spectral energy distribution can be built from broad band photometry. An example of a full broad band SED (from far-infrared to the X-rays) for an AGN in the XMM-COSMOS survey is presented in Figure REF  (see also Figure 1 in Chapter 1). The various lines represent the different models adopted to describe the data. Specifically, the best-fit starburst in the far-infrared (with a peak emission at \(\lambda \simeq 50-80\,\mu \) m), hot-dust from reprocessed AGN emission (\(\lambda \simeq 10\,\mu \) m), host-galaxy (\(\lambda \simeq 1\,\mu \) m), and the empirical disc templates (\(\lambda \simeq 0.2-0.6\,\mu \) m), respectively (see [6]} for further details on the SED modelling). The rest-frame 2500 Å and 2 keV are marked, as well as a representation of the \(\alpha _{\rm ox}\)  index for this object.
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